Exhaust gases of internal combustion engines of automobiles that use gasoline as fuel include harmful components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), and therefore, it is necessary to simultaneously purify and emit the respective harmful components using a redox reaction. For example, it is necessary to perform purification by converting hydrocarbons (HC) into water and carbon dioxide through oxidation, converting carbon monoxide (CO) into carbon dioxide through oxidation, and converting nitrogen oxides (NOx) into nitrogen through reduction.
As catalysts for treating the exhaust gases from such an internal combustion engine (hereinafter, referred to as “exhaust gas purifying catalyst”), three way catalysts (TWC) that can oxidize and reduce CO, HC and NOx have been used.
Regarding this kind of three way catalysts, for example, there is known a catalyst in which a precious metal is supported on a refractory oxide porous material such as an alumina porous material having a large surface area, and this is supported on a substrate, for example, a monolithic substrate having a refractory ceramic or metallic honeycomb structure, or is supported on refractory particles.
Since the bonding force between a precious metal as a catalytically active component and a substrate is not very strong, even if it is attempted to have the precious metal directly supported on the substrate, it is difficult to secure a sufficient supporting amount. Thus, in order to have a sufficient amount of a catalytically active component supported on the surface of a substrate, supporting of precious metals on a carrier having a high specific surface area has been implemented.
Regarding this kind of carrier, porous materials formed from refractory inorganic oxides such as silica, alumina and titania compounds have been traditionally known. Furthermore, in recent years, attention has been paid to apatite type composite oxides, which can provide carriers that have excellent heat resistance and can prevent sintering of the supported metal catalyst particles.
In regard to carriers formed from apatite type composite oxides, for example, Patent Document 1 (JP H7-24323 A) discloses a carrier formed from an apatite compound represented by formula: M10.(ZO4)6.X2 (wherein some or all of M's represent one kind or two or more kinds of transition metals selected from the elements of Group 1B and/or Group 8 of the Periodic Table, and preferably one kind or two or more kinds of transition metals selected from copper, cobalt, nickel, and/or iron, while these transition metals are included at a proportion of 0.5 wt % to 10 wt %; Z represents a trivalent to heptavalent cation; and X represents a monovalent to trivalent anion).
In Patent Document 2 (JP 2007-144412 A), as a catalyst with which the effect of purifying exhaust gas is achieved even in a relatively low temperature state and the purification performance as a three way catalyst is achieved even in a high temperature region, there is disclosed a catalyst for exhaust gas purification, which is composed of a composite oxide represented by (Laa-xMx) (Si6-yNy)O27-z and a precious metal component that forms a solid solution with the composite oxide or is supported thereon, and has high low-temperature activity and excellent heat resistance so that stabilized exhaust gas purification performance can be obtained therewith.
Patent Document 3 (JP 2011-16124 A) discloses a catalyst for exhaust gas purification, which is composed of a composite oxide represented by formula: (Aa-w-xMwM′x) (Si6-yNy)O27-z (wherein A represents a cation of at least one element of La and Pr; M represents a cation of at least one element of Ba, Ca and Sr; M′ represents a cation of at least one element of Nd, Y, Al, Pr, Ce, Sr, Li and Ca; N represents a cation of at least one element of Fe, Cu and Al; 6≤a≤10; 0<w<5; 0≤x<5; 0<w+x<5; 0<y ≤3, 0≤≤z≤3; A≠M′; and when A is a cation of La, x≠0), and a precious metal component that forms a solid solution with the composite oxide or is supported thereon.
However, three way catalysts including a catalyst layer composed of two layers or more layers have been hitherto suggested. These catalysts are being used for the purpose of assigning different functions to the respective catalyst layers, or of separating those components that have deteriorated activity when incorporated together in a same layer, into various layers.
For example, Patent Document 4 (JP 2-56247 A) discloses a low-temperature HC trap catalyst configured to include a catalyst layer composed of an upper layer and a lower layer on a honeycomb carrier, in which the lower layer is a HC adsorbent layer containing zeolite as a main component, and the upper layer is a purifying catalyst layer supporting a catalytic metal. With this catalyst, exhaust gas that has entered into the carrier cell passages diffuses into the HC adsorbent layer by passing through the upper purifying catalyst layer from the surface on the cell passage side to the surface on the HC adsorbent layer of the lower layer, and as a result, HC in the exhaust gas is adsorbed to the zeolite that constitutes the HC adsorbent layer at a low temperature. Furthermore, when the temperature of the catalyst, more particularly the temperature of the HC adsorbent layer, increases to a high temperature to a certain extent, for example, 120° C. to 200° C., along with the increase in the exhaust gas temperature, HC that has been adsorbed begins to escape, and the HC passes through the purifying catalyst layer of the upper layer from the lower HC adsorbent layer to the cell passage side and then flows out to the outside of the carrier cell passages. At that time, when the HC passes through the purifying catalyst layer, the HC is purified by being oxidized to water (H2O) or carbon dioxide (CO2) by the catalytic action of the catalyst metal.
Patent Document 5 (JP 2004-298813 A) discloses a layered catalyst including a carrier formed of a ceramic or a metal material; a first catalyst layer formed on the carrier; and a second catalyst layer formed on the first catalyst layer, in which the first catalyst layer is formed from a composite ceramic containing platinum-supported alumina having a platinum component supported on porous alumina, and an oxygen-storable ceria-zirconia composite oxide; and the second catalyst layer is formed from a composite ceramic containing at least one of a rhodium-supported ceria-zirconia composite oxide and a rhodium-supported alumina, which are obtained by supporting a rhodium component on a less heat-deteriorative ceria-zirconia composite oxide or porous alumina, and at least one of porous alumina and a less heat-deteriorative ceria-zirconia composite oxide.
When a catalyst layer composed of two layers or more layers is formed, there is a problem that it is difficult for the exhaust gas to diffuse into the interior of the catalyst layer on the substrate side. In order to solve such a problem, suggestions as follows have been made.
For example, Patent Document 6 (JP 2006-110485 A) discloses an exhaust gas purifying catalyst which increases the catalytic efficiency by increasing the gas diffusibility of exhaust gas in the catalyst layer, the exhaust gas catalyst including at least a carrier and plural layers formed on the carrier, in which at least one layer of the plural layers contains a catalyst component, at least one layer of the plural layers contains a catalyst component and also has pores in the layer, and the average diameter of the pores is from 0.2 μm to 500 μm.
Patent Document 7 (JP 2010-201362 A) suggests a catalyst carrier which can secure sufficient gas diffusibility and can suppress the grain growth of the catalyst metal, the catalyst carrier including a silicon carbide-based porous structure having a spongy three-dimensional skeleton part formed of a silicon carbide-based ceramic and a continuous pore part formed between the three-dimensional skeleton; a metal silicon layer formed on the surface of the three-dimensional skeleton part; and a SiO2 layer formed as a result of at least partial oxidation of the metal silicon layer.
Patent Document 8 (JP 2009-165929 A) discloses a catalyst for exhaust gas purification having a HC adsorbent layer provided on a honeycomb carrier; and a purifying catalyst layer laminated on this HC adsorbent layer, in which in order to increase the proportion the exhaust gas that passes through the purifying catalyst layer of the upper layer and diffuses to the HC adsorbent layer of the lower layer, to increase the amount of HC that is adsorbed to the HC adsorbent layer of the lower layer, and to increase the HC purification ratio while it is cold, plural fine passages having an average diameter larger than the average diameter of the particles that constitute the purifying catalyst layer, are formed in a dispersed manner in the purifying catalyst layer such that passages through which HC in the exhaust gas flowing through carrier cell passages can pass from the surface on the cell passage side of the purifying catalyst layer to the surface on the HC adsorbent layer side are generated.